JPA很多人都有额外的专栏 [英] JPA many to many with extra column

问题描述

我有一个我需要解决的问题。
核心问题是我想在JPA中为JoinTable for ManyToMany关系添加额外的列。在我的情况下，我有以下实体。

I have a following problem that I need to solve. The core issues is that I want to add additional column into JoinTable for ManyToMany relation in JPA. In my case I have following entities.

主题是一个简单的实体，有许多RemoteDocument（一个RemoteDocument可能被许多主题引用，因此它应该是ManyToMany关系） 。此外，RemoteDocument实体是只读的，因为它可能只能从Oracle物化视图中读取，而且禁止更改此物化视图。所以我想存储与某些主题相关的RemoteDocuments的顺序。事实上，我可以使用其他实体做类似的事情：

The Topic is a simple entity which has many RemoteDocument's (one RemoteDocument may be refered by many Topic's, hence it should be ManyToMany relation). Also RemoteDocument entity is read only because it may be read only from Oracle Materialized View moreover any altering of this Materialized View is forbidden. So I want to store order of RemoteDocuments related to some Topic. In fact I can do something like that with additional entity:

@Entity
public class Topic {
 @Id
 private Long id;
 @Basic
 private String name;

    @OneToMany
 private Set<TopicToRemoteDocument> association;
}

@Entity
public class RemoteDocument {
 @Id
 private Long id;
 @Basic
 private String description;
}

@Entity
public class TopicToRemoteDocument {
 @OneToOne
 private Topic topic;
 @OneToOne
 private RemoteDocument remoteDocument;
 @Basic
 private Integer order;
}

在这种情况下，附加实体TopicToRemoteDocument帮助我用OneToMany替换ManyToMany关联并添加额外的字段顺序。

In this case additional entity TopicToRemoteDocument helps me to replace ManyToMany association with OneToMany and add extra field order.

但是我想拥有ManyToMany关系但是在连接表中配置了额外的列

But I want to have ManyToMany relation but with configured additional column in join table

推荐答案

使用list而不是set，以及 @OrderColumn 注释，JPA将自动处理订单：

Use list instead of set, together with the @OrderColumn annotation and JPA will automatically take care of the order:

@MappedSuperclass
public class BaseEntity{

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    public Long getId(){
        return id;
    }

    public void setId(final Long id){
        this.id = id;
    }

}

@Entity
public class Topic extends BaseEntity{

    @ManyToMany(mappedBy = "topics")
    @OrderColumn
    private List<Document> documents = new ArrayList<Document>();

    public List<Document> getDocuments(){
        return documents;
    }

    public void setDocuments(final List<Document> documents){
        this.documents = documents;
    }

}

@Entity
public class Document extends BaseEntity{

    @ManyToMany
    @OrderColumn
    private List<Topic> topics = new ArrayList<Topic>();

    public List<Topic> getTopics(){
        return topics;
    }

    public void setTopics(final List<Topic> topics){
        this.topics = topics;
    }

}

生成的DDL（使用hibernate和HSQL）：

create table Document (
    id bigint generated by default as identity (start with 1),
    primary key (id)
);

create table Document_Topic (
    documents_id bigint not null,
    topics_id bigint not null,
    topics_ORDER integer not null,
    documents_ORDER integer not null,
    primary key (documents_id, topics_ORDER)
);

create table Topic (
    id bigint generated by default as identity (start with 1),
    primary key (id)
);

alter table Document_Topic 
    add constraint FK343B5D0B481100B2 
    foreign key (documents_id) 
    references Document;

alter table Document_Topic 
    add constraint FK343B5D0B558627D0 
    foreign key (topics_id) 
    references Topic;

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